Process for the recovery of low molecular weight C2+ hydrocarbons from a cracking gas

ABSTRACT

The invention relates to a process for the recovery of low molecular weight C 2+  hydrocarbons, in particular ethylene and ethane from a cracking gas, in particular from a fluid catalytic cracking waste gas. In accordance with the invention the light C 2+  hydrocarbons are scrubbed out of the cracking gas by absorption, using an organic, preferably paraffinic, physically acting scrubbing agent, the scrubbing agent having a molecular weight of between 50 and 75 g/mol, preferably between 60 and 75 g/mol. Particularly suitable as scrubbing agents are pentane, isopentane or mixtures thereof. Prior to the generation of the loaded scrubbing agent, co-extracted methane is advantageously stripped off.

The invention relates to a process for the recovery of low molecularweight C₂₊ hydrocarbons, in particular ethylene and ethane, from a gasmixture obtained by the cracking of hydrocarbons and having been freedof high molecular weight hydrocarbons.

In various cracking gas processes, in particular in fluidized catalyticcracking (FCC) a gas mixture is obtained which comprises, inter alia,low molecular weight hydrocarbons, in particular ethylene and ethane,but also C₃ and C₄ hydrocarbons. In most refineries the high molecularweight hydrocarbons (C₅₊ hydrocarbons) are first separated from the FCCwaste gas by fractional distillation. Thereafter, the C₃₊ hydrocarbonsare recovered by oil scrubbing, whereas the entire C₂ hydrocarbons, butalso a certain proportion of C₃₋ and C₄ hydrocarbons are discharged intothe fuel gas grid.

In the event that C₂ hydrocarbons as well are to be recovered from theFCC waste gas, this is done by partial condensation in a low temperatureprocess. Such a process is known for example from EP-B 0 185 202.Because the FCC waste gases invariably contain traces of higher,polyunsaturated hydrocarbons, nitrogen oxides and oxygen, this processalways involves a risk that explosive resins may form in the very lowtemperature items of equipment. A plant with partial condensation in alow temperature process accordingly always entails a certain safetyrisk. Thus, for example in February, 1990, such a plant exploded inFrance after having been in operation for eight years. Accordingly, itis an object of the present invention to provide a process of the typeas set out in the introduction, wherein the C₂₊ hydrocarbons can berecovered from the input gas in a simple manner, but where the safety ofthe plant is warranted by a special mode of operation.

This object is attained according to the invention in that the lowmolecular weight C₂₊ hydrocarbons are scrubbed from the gas mixture byabsorption by means of an organic, preferably paraffinic, physicallyacting scrubbing agent in an absorption column, the scrubbing agenthaving a molecular weight between 50 and 75 g/mol, preferably between 60and 75 g/mol, the scrubbing agent being passed in a circuit to be firstloaded with low molecular weight hydrocarbons, being regenerated in aregenerating column and subsequently being returned into the absorptioncolumn for renewed loading.

The process according to the invention offers the advantage that the lowmolecular weight hydrocarbons are removed from the gas mixture duringgas scrubbing, thereby avoiding a safety risk in that the formation ofexplosive resins is prevented by this procedure.

Advantageously, a C₄₊ hydrocarbon fraction, preferably pentane,isopentane or mixtures thereof, is employed as a scrubbing agent. Such ascrubbing agent is characterized by a very high solvent power for C₂hydrocarbons, in particular ethylene and ethane, but also for otherreadily soluble components. Light gasoline comprising proportions withinthe above described molecular weight range, in particular pentanecompounds are obtained during the pretreatment of the feed gas, forexample in the fractional distillation, and are accordingly readilyavailable.

In a preferred embodiment of the process according to the invention theloaded scrubbing agent, prior to its being fed into the regenerationcolumn, is stripped with a stripping gas, resulting in particular in thestripping off of co-absorbed methane. The methane thereby recovered canbe further processed or may, for example, be used as a fuel gas.

Particular advantages result if the stripping of the loaded scrubbingagent is conducted in a lower section of the absorption column. Thisprocedure, apart from simplifications in respect of apparatusrequirements, offers the advantage that methane dissolved in thescrubbing agent can even be stripped off from the scrubbing agent in theabsorption region in addition to the stripping region, since thestripping gas can flow through the entire absorption column.

Further advantages result from using evaporated scrubbing agent as astripping gas. For this purpose, heat is introduced into the lowerregion of the absorption column, for example by a boiling vessel,causing a certain part of the scrubbing agent to evaporate. Due to thelower temperatures in the upper region of the absorption column, theevaporated scrubbing agent is condensed once again and can be reloadedwith low molecular weight hydrocarbons. Stripping with a foreignstripping gas might result in contamination of the ethylene productflow.

The gas mixture is advantageously fed into the absorption column at apressure of 4 to 50 bar, preferably 10 to 30 bar. Although an elevatedpressure during absorption entails increased energy expenditures, italso substantially decreases the amount of required scrubbing agent.Thus, for example, the scrubbing agent throughput required may bereduced by half if the pressure is increased from 4 bar to 30 bar.

Advantageously, the absorption is carried out at temperatures between 0°and -50° C., preferably between -30° and -50° C., particularlypreferably between -35° and -45° C. Although even lower temperaturesincrease the solvent power of the scrubbing agent for the low molecularweight hydrocarbons, they, on the other hand, may cause a possibleincrease of NO₂ formation. Therefore, when employing feed flows having arelatively high NO_(x) content, a comparatively high temperature for theabsorption should be selected. The cold overhead product of theabsorption column can be employed for cooling the cracking gas mixture.

After the components lighter than ethylene, for example methane, havebeen stripped off the loaded scrubbing agent, the scrubbing agent loadedwith C₂₊ hydrocarbons is passed into a regenerating column. Theregenerating column is operated at temperatures higher than during theabsorption. The highest temperatures in the process according to theinvention prevail in the lower region of the regenerating column. Therethe scrubbing agent is heated preferably by means of a boiling vessel inorder to obtain desorption of the C₃₋ hydrocarbons from the scrubbingagent. The temperatures in the sump of the regenerating column are thusadvantageously so selected that 120° C., preferably 105° C. is notexceeded. The reason is that it was found that polymerization forexample to form butadiene will then be avoided. It is difficult toremove polymerization products from the scrubbing agent. Accordingly,the temperature in the sump region of the regenerating column ispredetermined at the lower limit by an inadequate desorption of the C₃₋hydrocarbons from the scrubbing agent and at the upper limit byincipient polymerization.

In principle, it is possible for the regeneration of the scrubbing agentto be performed at a pressure which is lower or higher than that for theabsorption. However, preferably the pressure in the regenerating columnis below that of the absorption column.

From the head of the regenerating column a gas flow is withdrawn, partlycondensed, passed into a separator and the condensate reintroduced fromthe separator as reflux into the regenerating column for reducing thelosses of scrubbing agent. In addition, a product gas flow rich in C₂/C₃ hydrocarbons is recovered by the separator. C₄₊ hydrocarbons passedinto the regeneration are returned from the sump of the regeneratingcolumn to the absorption column jointly with or serving as regeneratedscrubbing agent.

In accordance with a further development of the process according to theinvention, the head product of the regenerating column is partlycondensed and after the condensate has been separated and returned intothe regenerating column, the C₂ and C₃ hydrocarbon-rich gas flow issubjected to a C₂ /C₃ separation. In the C₂ /C₃ separation a C₂hydrocarbon flow is recovered which in a further separating stage isseparated into ethylene- and ethane-containing product gas flow.

In the following the invention will be further elucidated with referenceto a working example.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a process according to the invention includingabsorption, regeneration and further separation of the C₂ and C₃hydrocarbons by distillation.

It is to be noted that the totals of the data listed in the individualtables concerning the composition of individual flows, due toapproximations may exceed 100%.

By way of duct 1 an FCC cracking waste gas, freed by fractionaldistillation of C₄₊ hydrocarbons and dried, is fed into the absorptioncolumn 2 at a pressure of 13.3 bar and a temperature of 22° C. In thiscase the FCC waste gas has the following composition:

    ______________________________________                                        H.sub.2            3.4    Weight %                                            N.sub.2            11.8   Weight %                                            CO                 1.1    Weight %                                            CH.sub.4           31.6   Weight %                                            C.sub.2 H.sub.4    20.5   Weight %                                            C.sub.2 H.sub.6    28.4   Weight %                                            C.sub.3 H.sub.6    2.0    Weight %                                            C.sub.3 H.sub.8    1.0    Weight %                                            other              <0.1   Weight %                                            components                                                                    ______________________________________                                    

By way of duct 3 regenerated scrubbing agent composed to about threequarters of pentane and to about one quarter of isopentane is fed intothe upper region of the absorption column 2 at a temperature of -40° C.and a pressure of 13.1 bar. Above the chimney tray 4 the scrubbing agentis withdrawn, cooled in the side cooler 5 in order to withdraw theprocess heat formed and once again introduced into the absorptioncolumn. Above the chimney tray 6 loaded scrubbing agent is warmed up byindirect heat exchange (7) with regenerated scrubbing agent and returnedagain into the absorption column 2. Above the chimney tray 8 thescrubbing agent is once again withdrawn and further heated in the boilervessel 9, before once again being fed into the absorption column 2underneath the chimney tray 8. Above the chimney tray 10 the scrubbingagent is once again withdrawn and further heated by indirect heatexchange (11) with regenerated scrubbing agent and is fed into thelowermost section of the absorption column 2. The three column sectionsbelow the inlet of feed duct 1 serve for the removal of componentslighter than ethylene from the scrubbing agent. By the heating of thescrubbing agent, scrubbing agent vapor is formed which is employed asstripping gas for stripping off the components which are lighter thanethylene, in particular methane. Overhead of the absorption column 2 agas flow is thus withdrawn which is virtually free of C₂₊ hydrocarbonsand which has the following composition:

    ______________________________________                                        H.sub.2               6.7    Weight %                                         N.sub.2               23.0   Weight %                                         O.sub.2               0.1    Weight %                                         CO                    2.2    Weight %                                         CH.sub.4              61.6   Weight %                                         and traces of C.sub.2+ hydrocarbons.                                          ______________________________________                                    

From the sump of the absorption column 2 a scrubbing agent loaded withC₂₊ hydrocarbons and having a temperature of 54° C. and a pressure of13.4 bar is withdrawn:

    ______________________________________                                        C.sub.2 H.sub.4    3.4    Weight %                                            C.sub.2 H.sub.6    5.2    Weight %                                            C.sub.3 H.sub.6    0.8    Weight %                                            C.sub.3 H.sub.8    0.3    Weight %                                            C.sub.5 H.sub.12   65.6   Weight %                                            C.sub.5 H.sub.12 (Iso)                                                                           24.6   Weight %                                            other              <0.1   Weight %                                            components                                                                    ______________________________________                                    

The loaded scrubbing agent from duct 12 is further heated by indirectheat exchange (13) with indirect scrubbing agent and introduced into theregenerating column 14. In the regenerating column 14 the hotregeneration of the scrubbing agent takes place. For this purpose,regenerated scrubbing agent from the sump of the regenerating column(15) is partly (16) heated in the boiling vessel 17 to 105° C. andreturned into the lower region of the regenerating column. The scrubbingagent vapor thereby formed strips the C₂ and C₃ hydrocarbons from thescrubbing agent. The overhead product of the regenerating column (18)having a temperature of 57° C. and a pressure of 7.6 bar is cooled in acondenser 19 and introduced into a separator 20. Components which havecondensed out, for the predominant part scrubbing agent which hascondensed out, are returned from the separator 20 via line 21 into theupper region of the regenerating column 14. The scrubbing agent flowwithdrawn from the sump of the regenerating column 14 by way of duct 15at a temperature of 102° C. and a pressure of 7.8 bar and which is notreturned by way of duct 16 into the regenerating column is raised bypump 23 to a pressure of 14 bar and cooled in the heat exchangers 13,11, 7 and 39 to -40° C. The composition of the regenerated scrubbingagent withdrawn from the regenerating column is as follows:

    ______________________________________                                        C.sub.2 H.sub.6    0.4    Weight %                                            C.sub.3 H.sub.6    0.5    Weight %                                            C.sub.3 H.sub.8    0.2    Weight %                                            C.sub.5 H.sub.12   72.0   Weight %                                            C.sub.5 H.sub.12 (Iso)                                                                           27.0   Weight %                                            other              <0.1   Weight %                                            components                                                                    ______________________________________                                    

By way of duct 22 a small amount of scrubbing agent purge is separatedoff. A corresponding amount of scrubbing agent is added to the cooledscrubbing agent in duct 3 by way of duct 24.

By the separator 20 a C₂ -/C₃ hydrocarbon gas mixture is recovered inthe duct 25 having the following composition:

    ______________________________________                                        C.sub.2 H.sub.4    38.7   Weight %                                            C.sub.2 H.sub.6    55.2   Weight %                                            C.sub.3 H.sub.6    3.7    Weight %                                            C.sub.3 H.sub.8    1.9    Weight %                                            C.sub.5 H.sub.12   0.2    Weight %                                            C.sub.5 H.sub.12 (Iso)                                                                           0.4    Weight %                                            other              <0.1   Weight %                                            components                                                                    ______________________________________                                    

This gas flow is subsequently passed to a fractional distillation andfor this purpose is first introduced into column 26 for a C₂ -/C₃hydrocarbon separation. Column 26 is equipped with sump heating 27. Fromthe sump of the column 26 a distillate (LPG) which is essentiallycomposed of C₃ hydrocarbons is withdrawn by way of duct 28, with thefollowing composition:

    ______________________________________                                        C.sub.3 H.sub.6    58.8   Weight %                                            C.sub.3 H.sub.8    30.9   Weight %                                            C.sub.4 H.sub.6    0.3    Weight %                                            C.sub.5 H.sub.12   3.2    Weight %                                            C.sub.5 H.sub.12 (Iso)                                                                           6.7    Weight %                                            other              <0.1   Weight %                                            components                                                                    ______________________________________                                    

The overhead product of C₂ hydrocarbons having the followingcomposition:

    ______________________________________                                        C.sub.2 H.sub.4    36.9   Weight %                                            C.sub.2 H.sub.6    62.9   Weight %                                            C.sub.3 H.sub.6    0.1    Weight %                                            other              <0.1   Weight %                                            components                                                                    ______________________________________                                    

is fed by way of ducts 29 into the column 30. In order to keep the C₃ H₆C₃ H₈₊ components in duct 29 as low as possible, the flow 32 withdrawnabove the side tray 31 is returned as reflux into the upper region ofthe column 26. The reflux 32 has the following composition:

    ______________________________________                                        C.sub.2 H.sub.4    24.6   Weight %                                            C.sub.2 H.sub.6    75.3   Weight %                                            other              <0.1   Weight %                                            components                                                                    ______________________________________                                    

Column 30 serves for splitting the C₂ compounds. For this purpose, thesump region of column 30 is heated by way of the sump heating means 33while the overhead product is partly condensed (34), and the condensatewithdrawn by a separator 35 is returned as reflux into the column 30 byway of duct 36, there to attain a back-washing of the components whichare heavier than ethylene. More than 99.9% of the overhead product ofcolumn 30 is represented by ethylene, so that by way of duct 37 a pureethylene product flow with minimal contaminations is recovered. A sumpproduct having the composition:

    ______________________________________                                        C.sub.2 H.sub.4    0.9    Weight %                                            C.sub.2 H.sub.6    98.8   Weight %                                            C.sub.3 H.sub.6    0.3    Weight %                                            other              <0.1   Weight %                                            components                                                                    ______________________________________                                    

is withdrawn by way of duct 38 from the sump of the column 30. Thisethane fraction from duct 38 may for example be mixed with the overheadproduct of the absorption column 2 in duct 40 and be used as a fuel gas.

The columns 26 and 30 may be combined in a single column in which casefrom the head of this column likewise an ethylene flow is recovered,whereas from the sump of the column a C₃ hydrocarbon/ethane mixture iswithdrawn.

We claim:
 1. A process for recovery of low molecular weight C₂₊hydrocarbons, comprising ethylene and ethane, from a gas mixtureobtained by the cracking of hydrocarbons and having been freed of highmolecular weight hydrocarbons, said process comprising:scrubbing lowmolecular weight C₂₊ hydrocarbons from said gas mixture by absorptionwith an organic physically acting scrubbing agent in an absorptioncolumn, said scrubbing agent having a molecular weight between 50 and 75g/mol, wherein said scrubbing agent is passed in a circuit whereby thescrubbing agent is first loaded with low molecular weight hydrocarbons,then regenerated in a regenerating column and subsequently returned intosaid absorption column for renewed loading, wherein the loaded scrubbingagent, prior to being fed into said regenerating column, is strippedwith a stripping gas, whereby co-absorbed methane is stripped off.
 2. Aprocess according to claim 1, wherein a C₄₊ hydrocarbon fraction isemployed as the scrubbing agent.
 3. A process according to claim 2,wherein the scrubbing agent is pentane, isopentane, or mixtures thereof.4. A process according to claim 1, wherein stripping of said loadedscrubbing agent is carried out in a lower section of said absorptioncolumn.
 5. A process according to claim 4, wherein evaporated scrubbingagent is employed as said stripping gas.
 6. A process according to claim1, wherein evaporated scrubbing agent is employed as said stripping gas.7. A process according to claim 1, wherein said gas mixture is fed intosaid absorption column at a pressure of 4 to 50 bar.
 8. A processaccording to claim 7, wherein the pressure is 10-30 bar.
 9. A processaccording to claim 1, wherein the absorption is carried out attemperatures between 0° and -50° C.
 10. A process according to claim 9,wherein the adsorption temperature is between -30° and -50° C.
 11. Aprocess according to claim 9, wherein the adsorption temperature isbetween -35° and -45° C.
 12. A process according to claim 1, wherein thetemperature in the sump of said regenerating column does not exceed 120°C.
 13. A process according to claim 12, wherein the temperature in thesump does not exceed 105° C.
 14. A process according to claim 1, whereinthe pressure in said regenerating column is below that of saidabsorption column.
 15. A process according to claim 1, wherein gascontaining C₂ hydrocarbons and C₃ hydrocarbons is withdrawn from thehead of said regenerating column, subjected to partial condensation andseparated from condensate, and then subjected to a C₂ /C₃ separation,andC₂ hydrocarbons are withdrawn from the C₂ /C₃ separation andseparated further into ethylene- and ethane-containing product gasflows.
 16. A process according to claim 1, wherein the scrubbing agentis paraffinic.
 17. A process according to claim 1, wherein the molecularweight of the scrubbing agent is between 60 and 75 g/mol.
 18. A processaccording to claim 1, further comprising withdrawing a gas flow from thehead of said regenerating column, subjecting said gas flow to partialcondensation, introducing the partially condensed gas flow into aseparator wherein said partially condensed gas flow is separated into acondensate and a C₂ /C₃ hydrocarbon product gas flow, and saidcondensate is reintroduced to said regenerating column as reflux.
 19. Aprocess according to claim 18, wherein said C₂ /C₃ hydrocarbon productgas flow is subjected to fractional distillation from which a C₃hydrocarbon product stream is obtained as a bottom stream and a C₂hydrocarbon product stream is obtained as an overhead stream.
 20. Aprocess according to claim 19, wherein said C₂ hydrocarbon productstream is delivered to a column for splitting of the C₂ hydrocarbonsfrom which an ethylene product stream is obtained as an overhead streamand an ethane product stream is obtained as a bottom stream.
 21. Aprocess according to claim 1, wherein said gas mixture is a waste gasfrom a fluidized catalytic cracking unit and wherein, prior tointroduction into said absorption column, C₄₊ hydrocarbons are removedfrom said waste gas by fractional distillation.
 22. A process for therecovery of C₂₊ hydrocarbons from a gas mixture containing ethylene andethane, said process comprising:introducing said gas mixture into anabsorption/stripper column, wherein a scrubbing agent is introduced intosaid absorption/stripper column at a point above the introduction ofsaid gas mixture, and said scrubbing agent is removed from saidabsorber/stripper column at at least one point below the point ofintroduction of said gas mixture, heated and reintroduced into saidabsorber/stripper column at a point below the introduction of said gasmixture, whereby the scrubbing agent vapor resulting from the heating ofthe removed scrubbing agent acts as a stripping gas for stripping offco-absorbed methane from said scrubbing agent; removing loaded scrubbingagent from the bottom of said absorption/stripper column, heating saidloaded scrubbing agent by indirect heat exchange, and introducing theheated, loaded scrubbing agent into a regeneration column; removingregenerated scrubbing agent from the bottom of said regeneration columnand recycling said regenerated scrubbing agent to the top of saidabsorption/stripper column; and removing a C₂ /C₃ hydrocarbon gas streamfrom the head of said regeneration column, cooling said C₂ /C₃hydrocarbon gas stream in a condenser and delivering the resultantpartially condensed C₂ /C₃ hydrocarbon stream into a separator fromwhich a condensate stream is removed and reintroduced into the upperregion of said regeneration column and a product C₂ /C₃ hydrocarbon gasmixture containing ethane and ethylene is also removed from saidseparator.